1. Technical Field
This application relates generally to drill rods and methods of making and using such drill rods. In particular, this application relates to drill rods used in sonic core drilling processes.
2. Background and Relevant Art
In sonic core drilling processes, variable frequency vibration is created by an oscillator. The vibration is then mechanically transferred to the drill string of the core barrel and/or casing. The vibration is transmitted in an axial direction down through the drill string to an open-faced drill bit. As a result, the drill string may be rotated and/or mechanically pushed as it is vibrated into the sub-surface formation.
Often, sonic core drilling processes are used to retrieve a sample of material from a desired depth below the surface of the earth. In one conventional sonic drilling process, an open-faced drill bit is attached to the bottom or leading edge of a core barrel. The core barrel is attached to the bottom of a drill string, which is a series of individually threaded and coupled drill rods that are assembled section-by-section as the depth of the borehole increases. The top of the drill string is then coupled to a sonic drill head. The sonic drill head may include high-speed, rotating counterbalances that produce resonant energy waves and a corresponding high-speed vibration to be transmitted through the drill string to the core barrel. As a result, the sonic drill head can vertically vibrate the core barrel. In addition, the drill head can rotate and/or push the core barrel into the sub-surface formation to obtain a core sample. Once the core sample is obtained, the core barrel (containing the core sample) is retrieved by removing (or tripping) the entire drill string out of the borehole that has been drilled. Once retracted to the surface, the core sample may then be removed from the core barrel.
In addition, an outer casing with a larger diameter than the core barrel may be used to maintain an open borehole. Like the core barrel, the casing may attach to an open-faced drill bit at a lower end, and to a drill head at an upper end. The outer casing may be advanced and removed in the same manner as the core barrel. For example, the casing can be advanced into a formation one drill rod at a time using a drill head. Similarly, the casing can be removed by tripping the drill rods of the outer casing out of the borehole.
In contrast, in a sonic wireline drilling process, the core barrel and the casing are advanced together into the formation. The casing again has an open-faced drill bit and is advanced into the formation. However, the core barrel (inner tube) does not contain a drill bit or connect to a drill string. Instead, the core barrel mechanically latches inside of and at the bottom of the casing and advances into the formation along with the casing. When the core sample is obtained, a drill operator can retrieve the core barrel using a wireline system. Thereafter, the drill operator can remove the core sample from the core barrel at the surface, and then drop the core barrel back into the casing using the wireline system. As a result, the wireline system eliminates the time needed to trip the drill rods and drill string in and out of a borehole for retrieval of the core sample.
Both conventional and wireline sonic drilling processes include serious drawbacks. One of which, is that the drilled material at and ahead of the bit face has to be displaced. The bit face material will typically take the path of the least resistance. The displaced material can enter the core barrel, which can disturb, elongate, compact, and, in some cases, heat the core samples. The drilled material can be pushed outward into the formation, which can cause compaction of the formation and alter its natural state. Furthermore, the drilled material can enter the annular space between the outer casing and the borehole wall, which can cause increased friction and/or heat and bind the casing or core barrel in the borehole.
In addition, when drilling hard and/or dry formations, the displaced material often cannot move anywhere, so it is re-drilled numerous times creating additional heat, drilling inefficiencies, and bound/stuck casings. When a casing binds or sticks, the drilling process may be slowed or stopped altogether. In addition, a bound or stuck casing may require the use of a flushing medium, such as water, mud, or air, to remove the excess material and free up the casing. However, the addition of a flushing medium is often undesirable because it can cause sample disturbance and borehole contamination.